Aluminium/silicon disordering and melting in sillimanite at high pressures

Abstract

Naturally occurring polymorphs of Al₂SiO₅ (andalusite, kyanite and sillimanite) have assumed a special significance for geologists because of their value as indicators of the pressures (P) and temperatures (T) experienced by metamorphic rocks^1,2, and for materials scientists because the binary system Al₂SiO₅–Al₂O₃ contains the important refractory mineral, mullite³. The equilibrium stability field of sillimanite in natural rocks and the process by which it can be transformed to mullite for ceramic materials (mullitization) have received particular attention because of the role of cation ordering (Al³⁺/Si⁴⁺) in stabilizing these structures^1–7. Here we present the results of annealing experiments undertaken to determine the disordering behaviour of sillimanite crystals at high confining pressures. At P≃18–20 kbar and T≃1,300–1,700 °C, they undergo partial melting to give a SiO₂-rich melt. The residual crystals become progressively more disordered as they become enriched in Al₂O₃ until they transform to a higher-symmetry structure. Our observations shed new light on previous thermochemical studies of Al/Si order–disorder in sillimanite and on the phase relationships between sillimanite and mullite.